The present invention relates to a method of controlling local polarizations in a nonlinear optical material which is suitable for use in the formation of a periodically inverted-polarization structure in an optical second-harmonic generator (SHG).
Second-harmonic generators for converting a radiation having a fundamental frequency of .omega. into a radiation having a second-harmonic frequency of 2.omega. may be combined with a semiconductor laser, for example, to provide a short-wavelength radiation source for converting a near-infrared radiation into a blue radiation. The short-wavelength radiation source can increase a wavelength range available for the coherent radiation produced by the semiconductor laser, and is effective in giving semiconductor lasers a widened range of applications and optimizing laser applications in various fields of art. For example, a shortened laser wavelength allows optical or magnetooptical recording and reproducing devices which employ laser beams to increase the recording density and resolution.
The second-harmonic generators include a bulk-type second-harmonic generator made of KTP, a second-harmonic generator in the form of an optical waveguide employing a greater nonlinear optical constant, and a second-harmonic generator of the Cerenkov radiation type comprising an optical waveguide disposed on a single-crystal substrate made of a nonlinear optical material such as lithium niobate (LiNbO.sub.3), so-called "LN", for guiding near-infrared radiation to produce blue radiation as a second harmonic in a radiation mode from the single-crystal substrate.
The bulk-type second-harmonic generator has a relatively low SHG conversion efficiency because of its characteristics. The second-harmonic generator of the Cerenkov radiation type has practical problems because the second-harmonic beam is radiated in a direction within the substrate and the beam spot is of a special shape such as a crescent shape.
For high conversion SHG efficiency, it is necessary to confine and guide a fundamental and a second harmonic within the same waveguide and also to equalize the phase propagation velocities of the fundamental and the second harmonic.
One known process of equalizing the phase propagation velocities is a quasi-phase matching process which provides a periodically inverted-polarization structure having polarizations periodically inverted in the direction in which radiations are guided in an optical second-harmonic generator.
Such a periodically inverted-polarization structure may be realized by either cutting a crystal of single polarization into thin slices and joining the slices with their crystallographic axes being alternately inverted to periodically inverting the polarizations thereof, or controlling the polarity of a current which is supplied when a crystal is grown for thereby producing periodic polarizations or domains. The former process is disadvantageous in that it is difficult to achieve an optical coupling when the thin crystal slices are joined. The latter process is problematic because it requires a large-scale manufacturing apparatus and it is difficult to control the formation of inverted polarizations.
There has been proposed in Japanese laid-open patent publication No. 4-335620 a process of forming a periodically inverted-polarization structure by positioning first and second electrodes on a ferroelectric material which is divided into single domains, the first and second electrodes being arranged in the direction of polarization in a pattern corresponding to the pattern of an inverted- polarization structure with at least the first electrodes being finally obtained. In the disclosed process, a voltage ranging from 1 kV/mm to 100 kV/mm is applied between the first and second electrodes such that the negative side of spontaneous polarization of the ferroelectric material is maintained at a negative potential and the positive side at a positive potential.
One example of such a process of controlling the inversion of polarizations will be described with reference to FIG. 1 of the accompanying drawings. In FIG. 1, the direction of spontaneous polarization in a nonlinear optical material is indicated by the arrow "d", and the direction of polarizations in local inverted-polarization domains is indicated by the arrow "h".
A nonlinear optical material substrate 10 comprises a single crystal of LN having a thickness of about 0.1 mm which is divided into similar single domains in the direction of the c axis thereof. On the +c surface of the substrate 10, there are disposed a plurality of first electrodes 11 as parallel strips each having a width W of 1.3 .mu.m at a pitch P of 2.6 .mu.m, for example, in the same pattern as a pattern of desired local inverted-polarization domains each having a width of 1.3 .mu.m at a pitch P of 2.6 .mu.m, for example. A second electrode 12 is disposed on the entire -c surface of the substrate 10 which lies opposite to the +c surface. A power supply 5 for applying a voltage is connected between the first electrodes 11 and the second electrode 12.
At room temperature, a voltage of 26 kV/mm, for example, is applied by the power supply 5 between the first and second electrodes 11, 12 such that the first electrodes 11 on the positive side of spontaneous polarization of the nonlinear optical material 10 is maintained at a positive potential and the second electrode 12 on the negative side at a negative potential, thereby forming local inverted-polarization domains 14. The local inverted-polarization domains 14 are thus produced in the form of parallel strips at the pitch P, and the resultant overall structure becomes a periodically inverted-polarization structure for use as a second-harmonic generator.
If the width Wi of each of the local inverted-polarization domains 14 is 1/2 of the pitch P, then theoretically, the second-harmonic generator produces a greatest second-harmonic output and hence has a high SHG conversion efficiency.
FIG. 2 of the accompanying drawings illustrates in cross section a periodically inverted-polarization structure produced by the conventional process shown in FIG. 1, FIG. 2 being based on a microscopic photograph thereof. As shown in FIG. 2, in a region 1 of the periodically inverted-polarization structure, the width Wi of each of the local inverted-polarization domains 14 is greater than the width W of each of the electrodes 11. In a region 2, however, polarizations are inverted below only the edges of the electrodes 11, and local inverted-polarization domains are not formed uniformly below the electrodes 11. The periodically inverted-polarization structure composed of the local inverted-polarization domains 14 as shown in FIG. 2 is different from the periodically inverted-polarization structure which theoretically provides a high SHG conversion efficiency, and produces a lower second-harmonic output.